Method and device for controlling a multiphase resonant dc/dc converter, and corresponding multiphase converter

ABSTRACT

The control method according to the invention concerns a multiphase resonant DC/DC converter comprising a plurality of identical elementary DC/DC converters connected in parallel. Supply currents (IR1, IR2, . . . IRn) are measured in the elementary DC/DC converters in order to balance these same currents. According to the method, switching frequencies of the elementary DC/DC converters are controlled on the basis of the supply currents in such a way as to achieve the balance. Another feature involves setting the supply currents to a common reference intensity (Iref) determined on the basis of a difference between an output voltage (Vo) of the multiphase converter and a nominal voltage (Vref).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and a device for controlling amultiphase resonant DC/DC converter which is designed to supply from adirect current source power equipment with a plurality of storage unitsof a motor vehicle.

The invention also relates to the corresponding multiphase converter, aswell as to an AC/DC converter comprising a multiphase DC/DC converterprovided with a control device of this type.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

Resonant DC/DC converters which make it possible to transform onevoltage level into another are frequently implemented in highpower-density and high-performance DC/DC conversion systems.

A simplified basic architecture of a resonant direct current—directcurrent converter (or DC/DC according to the corresponding acronym) ofthe “LLC series” type (i.e. comprising two inductive resistors 1, 2 anda capacitor 3 in series) is represented in FIG. 1.

At the input, two field effect power transistors 4, 5 of the MOSFET typeare connected to a direct current source 6, forming a half-bridge with afirst, so-called high-side transistor 4 connected to the potentialterminal of the source 6, and a second, so-called low-side transistor 5connected to the earth.

The resonant circuit 7 comprises in series the capacitor 3 and a firstinductive resistor 1 which determines the resonance, and a secondinductive resistor 2 of a transformer 8. At the output there are tworectifier diodes 9 and a filtering capacitor 10 which supplies a loadresistor 11 with direct current.

The two power MOSFETs 4, 5 are switched in a complementary manneraccording to a duty cycle close to 50%, leaving a constant dead time inorder to avoid a phenomenon of simultaneous conduction.

This known LLC converter functions according to a zero voltage switchingmode of all the semiconductors 4, 5 in a wide range of charges, withimproved EMC (electromagnetic compatibility) performance and a limitedswitching frequency.

However, because of the substantial pulse currents on the secondary ofthe transformer 8 and high currents on the primary, the use of anelementary converter of this type is limited to low or medium powerlevels. In fact, applications which require high powers and strongcurrents lead to iron losses and additional switching losses whichreduce the global performance of the converters.

In order to produce high-power converters and eliminate thesedisadvantages, use is made of a multiphase architecture in which aplurality of identical elementary converters are connected at the inputin parallel on a single source 6, and at the output in parallel on asingle load 11 (parallel-parallel LLC multiphase converter), as shownclearly in FIG. 1, in order to share the total power amongst all theelementary converters and to distribute the currents better between thedifferent power units in order to obtain good performance.

The current which circulates in the primary of the transformer 8 can bereduced, and the current constraints imposed on the MOSFET transistors4, 5 are reduced and distributed between the different power units.

A well-known control method for the transistors 4, 5 of the half-bridgesof the elementary converters of the multiphase converter consists ofmaking the n phases function (n being equal to or more than 2) at acommon switching frequency, with a phase shift Δφ of T/n (period numberof units) between two adjacent elementary converters, in order to obtainan output current with fewer pulse transients.

However, this well-known control method is valid only in the hypothesisthat all the elementary converters have exactly the same electricalcharacteristics, i.e. same first induction coil L_(R1), L_(R2), L_(R3)at the resonance of the first inductive resistor 1, same capacityC_(R1), C_(R2), C_(R3) at the resonance of the capacitor 3, same secondinduction coil L_(M1), L_(M2), L_(M3) of the second inductive resistor2, and same switching parameters Q_(H1), Q_(H2), Q_(H3); Q_(L1), Q_(L2),Q_(L3) of the transistors 4, 5.

This means that a type of super-symmetry must be maintained between allthe elementary converters.

In the event of malfunctioning, the slightest dissymmetry is liable tocreate enormous balancing problems, with the largest fraction of thecurrent passing via a single power unit and leaving the other unitsfunctioning at a low output power, or even zero power.

For example, a difference of 5% between the first induction coil L_(R1)of the first inductive resistor 1 and another can introduce a currentimbalance which can reach 90%.

Since each parameter of the electronic component has a certain tolerance(typically in the motor vehicle industry ±5% for a capacitor and ±10%for an inductive resistor), this well-known control method is in facttotally inefficient for ensuring the balancing of the currents betweenthe different elementary converters, and therefore for maintainingacceptable global performance.

European patent application EP2299580 proposes a method and a device forcontrolling a multiphase resonant DC/DC converter in order to solve theproblem of imbalance of the currents in elementary converters of the LLCtype, without costly selection and matching of the components.

As represented schematically in FIG. 1, the method consists inparticular of measuring the supply currents of the elementary converters(three being represented) by means of shunts 12, and of controlling thephase shifts Δφ₁₋₂, Δφ₂₋₃ between the control signals, with the samecommon frequency, of the MOSFET transistors 4, 5 of the half-bridges,such as to balance these supply currents.

However, this method is close to the conventional method, and computersimulations carried out by the inventive body have shown that thismethod was not optimum. Ways therefore remain for improvement of acontrol method of the same type, making it possible to eliminate theabove-described disadvantages.

GENERAL DESCRIPTION OF THE INVENTION

For the purpose of applications in the highly competitive motor vehicleindustry, the objective of the present invention is consequently to makeprogress in this respect.

Specifically, the subject of the invention is a method for controlling amultiphase resonant DC/DC converter comprising a plurality of identicalelementary resonant DC/DC converters connected in parallel.

This method is of the type consisting of measuring each of the supplycurrents of the elementary converters in order to balance the supplycurrents.

The method according to the invention is distinguished in that it alsoconsists of controlling switching frequencies of the elementaryconverters according to the supply currents, so as to carry out thisbalancing.

Highly advantageously this method for controlling a multiphase resonantDC/DC converter also consists of setting the supply currents to a commonreference intensity which is determined according to a differencebetween an output voltage of the multiphase converter and a nominalvoltage.

In the method according to the invention, the supply currents arepreferably determined by measuring differences of potential at theterminals of shunts inserted in series in supply circuits of theelementary converters.

The switching frequencies are advantageously derived from avoltage—frequency conversion.

The invention also relates to a device for controlling a multiphaseresonant DC/DC converter comprising a plurality of identical elementaryresonant DC/DC converters connected in parallel.

This control device is of the type comprising intensity measurementmeans for each of the supply currents of the elementary converters, inorder to balance supply currents, and which can implement theabove-described method.

The control device according to the invention is distinguished in thatit additionally comprises frequency generators which generate switchingfrequencies for the elementary converters according to these supplycurrents.

In addition, this control device advantageously comprises:

-   -   a comparator between an output voltage of the multiphase        converter and a nominal voltage;    -   a regulation loop which sets the supply currents to a common        reference intensity.

Preferably, the intensity measurement means comprise:

-   -   shunts inserted in series in supply circuits of the elementary        converters;    -   voltage measurement means which determine differences of        potential at the terminals of these shunts.

The invention also relates to a multiphase resonant DC/DC convertercomprising a plurality of identical elementary resonant DC/DC convertersconnected in parallel, and comprising the above-described controldevice.

The elementary converters are advantageously of the LLC type, eachcomprising two inductive resistors and a capacitor.

Preferably, switching means of each of the elementary converters areconstituted by switching elements connected in the form of ahalf-bridge.

Benefit is derived from the fact that in the multiphase resonant DC/DCconverter according to the invention, a plurality of specimens of theelectronic components of the elementary converters are not matched.

The invention also relates to a multiphase resonant AC/DC converter,distinguished in that it is advantageously constituted by an AC/DCconverter at the input coupled to a multiphase resonant DC/DC converterwith the above specifications at the output.

These few essential specifications will have made apparent to personsskilled in the art the advantages provided by the method for controllinga multiphase resonant DC/DC converter according to the invention, aswell as by the corresponding control device and multiphase converter, incomparison with the prior art.

The detailed specifications of the invention are provided in thefollowing description in association with the appended drawings.

It should be noted that these drawings serve the purpose simply ofillustrating the text of the description, and do not constitute in anyway a limitation of the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents schematically a multiphase resonant DC/DC converterand its control device known in the prior art.

FIG. 2 is a schematic diagram of a device for controlling a multiphaseresonant DC/DC converter according to the invention.

FIG. 3 represents schematically a preferred embodiment of a multiphaseresonant DC/DC converter and its control device according to theinvention.

FIG. 4 represents schematically a multiphase resonant AC/DC converterand its control device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

As indicated in FIG. 2 the supply currents I_(R)', I_(R2), . . . I_(Rn)of the elementary converters of the multiphase resonant DC/DC converter(with n phases) according to the invention are detected by theirrespective shunts 12, and converted 13 into differences of potentialV_(R1), V_(R2), . . . V_(Rn).

These signals V_(R1), V_(R2), . . . V_(Rn) are filtered by low-passfilters 14 with high gain in the switching frequency band F₁, F₂, . . .F_(n) of the elementary converters, in order to eliminate the noiseprovided by the switching elements 4, 5.

These filters 14 normally comprise a common mode low-pass filter forfiltering of the common mode noise, and a differential mode low-passfilter for filtering of the differential mode noise. The order of thefilters 14, determining the gradient of the frequency response, matterslittle.

The signals filtered are amplified 14 to levels I_(m1), I_(m2), . . .I_(mn) which are suitable for the current regulation loops of theelementary converters.

In the voltage regulation loop which is implemented in the presentinvention, the output voltage V_(o) of the multiphase converter iscompared 15 with a nominal voltage V_(ref) and results in an errorsignal e_(v).

A voltage divider bridge is advantageously added for the measurement ofthe output voltage V_(o) according to the level of the output voltageV_(o) of the multiphase converter.

The nominal voltage V_(ref) is provided either by a constant or variableexternal voltage reference, or by an internal source, such as, forexample, a circuit of the TL431 type.

Regulation 16 implemented by the method according to the invention is ofany type, such as PI, PID, etc.

Electrical insulation 17 is always necessary, generally obtained bymeans of photodiodes. The electrical insulation stage 17 can be placedin any location of the voltage regulation loop, before the regulation 16or after a limiter stage 18.

The limiter stage 18 is designed to eliminate the deviating values of anintensity I_(ref) in order to avoid the risk of overloading and toimprove the robustness of the multiphase converter.

Control of the current level of each elementary converter is alsoadvantageous, or even obligatory, when the multiphase converter issituated between two voltage sources of an electric vehicle or a hybridvehicle (high-voltage battery and low-voltage battery).

A pass band of this voltage regulation loop is advantageouslyapproximately a few KHz.

In the preferred embodiments of the invention, the reference intensityI_(ref) is common to all the current regulation loops which regulate thesupply currents I_(R1), I_(R2), . . . I_(Rn).

The advantage of these current regulation loops is that two elementaryconverters can share a single input current supplied by the same source6, even if the electrical parameters of the elementary convertersdiffer, although their wiring diagrams are identical because of thedispersion of the characteristics of the components 1, 2, 3, 4, 5, 8.

Since the supply currents I_(R1), I_(R2), . . . I_(Rn) are the same,because the source 6 is the same, the power consumed is the same for allthe elementary converters, irrespective of the tolerances on theelectronic components.

In the preferred embodiments of the invention, error voltages e_(i1),e_(i2), . . . e_(in) corresponding to a regulated intensity I_(reg) 19derived from a comparison 20 between the reference intensity I_(ref) andthe supply currents I_(R1), I_(R2), . . . I_(Rn) are converted intoswitching frequencies F₁, F₂, . . . F_(n) by a voltage—frequencyconversion 21, in order to control the elementary converters.

Since the error voltages e_(i1), e_(i2), . . . e_(in) are not identicalif the circuits have dissymmetries, the elementary converters functionat different switching frequencies F₁, F₂, . . . F_(n).

In a multiphase converter according to the invention represented in FIG.3, the control units 21 for the switching elements 4, 5 of thehalf-bridges of the elementary converters generate complementary squaresignals with a duty cycle close to 50%, with a constant dead time inorder to avoid the phenomenon of overlapping, in a known manner.

However, unlike the prior art, no phase shift is introduced between thesquare signals which control the different elementary converters.

The control device for a multiphase converter previously described hasnumerous advantages, in particular:

-   -   simplicity of construction and implementation;    -   balancing of the supply currents I_(R1), I_(R2), . . . I_(Rn) of        the elementary converters independently from the tolerances of        the components;    -   zero voltage switching mode within the limit of the functioning        frequencies F₁, F₂, . . . F_(n);    -   regular distribution of the power between the elementary        converters, and consequently regular distribution of the losses        and temperature increases;    -   possibility of obtaining high power levels by putting a        plurality of high-performance elementary converters in parallel;    -   improvement of the robustness;    -   possible functioning between only two sources of voltage.

All of these advantages show that a multiphase resonant DC/DC convertercomprising a control device of this type is an excellent solution forhigh-power conversion systems.

FIG. 3 represents schematically a converter with n phases according tothe invention.

The elementary resonant DC/DC converters comprise cells 22 of the LLCtype, their inputs being connected in parallel on the same source 6, andtheir outputs also being connected in parallel with a filteringcapacitor 10 and a common charge resistor 11.

The resistors R₁, R₂, . . . R_(n) of the shunts 12 which constitute thecurrent sensors can vary between a few mΩ and a few hundred mΩ accordingto the supply currents I_(R1), I_(R2), . . . I_(Rn) and the level of themeasurement voltages V_(R1), V_(R2), . . . V_(Rn) required.

The shunts 12 are inserted in series on the earthing side in the supplycircuits of the elementary converters, such that the measurementvoltages V_(R1), V_(R2), . . . V_(Rn) are not floating.

Each elementary converter comprises a half-bridge consisting of twoswitching elements of the MOSFET type 4, 5.

Alternatively, the MOSFETs 4, 5 are replaced by switching elements ofthe BJT type (acronym for Bipolar Junction Transistor), or IGBT type(acronym for Insulated Gate Bipolar Transistor).

A plurality of switching elements 4, 5 of the same type areadvantageously grouped in parallel in order to decrease the losses byconduction and increase the intensities admissible.

Each LLC cell 22 comprises in series a first inductive resistor 1(resonant inductive resistor) with a first induction coil L_(R1),L_(R2), . . . L_(Rn), a capacitor 3 (resonant capacitor) with a capacityC_(R1), C_(R2), . . . C_(Rn), and a second inductive resistor 2(magnetising or primary inductive resistor) with a second induction coilL_(M1), L_(M2), . . . L_(Mn).

The resonant capacitor 3 is advantageously divided into two capacitiveelements with a value two times smaller which are connected in series,and connected in parallel on the half-bridge 4, 5, the mid-point beingconnected to the transformer 8.

The first inductive resistor 1 is represented as a separate component;alternatively, it is completely integrated in the transformer 8 and itis considered that it has a leakage inductance.

The second inductive resistor 2 is also represented as another separatecomponent; alternatively, it is also completely integrated in thetransformer 8.

Since the switching frequencies can be different, the capacitors C_(R1),C_(R2), . . . C_(Rn), the first induction coils L_(R1), L_(R2), . . .L_(Rn), and the second induction coils L_(M1), L_(M2), . . . L_(Mn) ofthese electronic components of the multiphase resonant DC/DC converterdo not need to be matched.

In elementary converter models with a low output voltage where thedirect voltage drop of the diodes 9 is too great to be neglected, thediodes 9 are advantageously diodes of the Schottky type, in order toreduce the load losses.

For the same purpose, use will highly advantageously made of synchronousrectifiers. The synchronous rectifiers in question comprisesemiconductor switches which are connected in parallel to the diodes 9,such that these switches are on when the diodes 9 are polariseddirectly.

As shown clearly in FIG. 3, all of the elementary converters of themultiphase converter according to the invention are controlled by acontrol module 23 which generates the switching frequencies F₁, F₂, . .. F_(n) of the control units 21 of the switching elements 4, 5 accordingto the measurement voltages V_(R1), V_(R2), . . . V_(Rn) and the outputvoltage V_(o) according to the schematic diagram in FIG. 2.

FIG. 4 shows another example of a multiphase converter in whichadvantage will be derived from implementation of the method and of thecontrol device according to the invention.

This is a multiphase alternating current—direct current (AC/DC)converter comprising:

-   -   an AC/DC converter 24 at the input which can be connected to an        alternative voltage source 25;    -   a filtering capacitor 26 at the output from the AC/DC converter;    -   a multiphase resonant DC/DC converter comprising a plurality of        elementary resonance converters 27 which are connected at the        input in parallel on the output of the AC/DC converter, and are        connected in parallel at the output;    -   a control module 23 which functions according to the principles        of the invention.

The charge 28 at the output is constituted by one or more pieces ofequipment, for example a battery 29 and a resistive charge 30.

This architecture will be advantageously implemented in an electricvehicle in order to charge the high-voltage batteries of the vehicle(for example 300 V DC) from the mains 25 (in 12 particular 220 V AC),and at the same time to charge the low-voltage battery 29 (for example12 V) with good performance.

It will be appreciated that the invention is not limited simply to thepreferred embodiments previously described.

The architecture of the elementary resonant DC/DC converters can bedifferent from that specified. In particular the resonant circuits (7,22) of the LLC series type can be replaced by circuits of the LCparallel or LC series type, or also by circuits of the LCC type.

The supply currents of the elementary converters I_(R1), I_(R2), . . .I_(Rn) can alternatively be measured by intensity measurement means 13which are different from shunts 12, for example by Hall effect sensorsor current transformers.

The invention thus incorporates all the possible variant embodiments,provided that these variants remain within the scope defined by thefollowing claims.

1. Method for controlling a multiphase resonant DC/DC convertercomprising a plurality of identical elementary resonant DC/DC convertersconnected in parallel of the type consisting of measuring each of thesupply currents (IR1, IR2, . . . IRn) of said elementary converters inorder to balance said supply currents (IR1, IR2, . . . IRn), wherein italso consists of controlling switching frequencies (F1, F2, . . . Fn) ofsaid elementary converters according to said supply currents (IR1, IR2,. . . IRn), so as to carry out said balancing.
 2. Method for controllinga multiphase resonant DC/DC converter according to claim 1, wherein italso consists of setting the supply currents (IR1, IR2, . . . IRn) to acommon reference intensity (Iref) which is determined according to adifference between an output voltage (Vo) of said multiphase converterand a nominal voltage (Vref).
 3. Method for controlling a multiphaseresonant DC/DC converter according to claim 1, wherein said supplycurrents (IR1, IR2, . . . IRn) are determined by measuring differencesof potential (VR1, VR2, . . . VRn) at the terminals of shunts (12)inserted in series in supply circuits of said elementary converters. 4.Method for controlling a multiphase resonant DC/DC converter accordingto claim 1, wherein said switching frequencies (F1, F2, . . . Fn) arederived from a voltage—frequency conversion (21).
 5. Device forcontrolling a multiphase resonant DC/DC converter comprising a pluralityof identical elementary resonant DC/DC converters connected in parallel,of the type comprising intensity measurement means (13) for each of thesupply currents (IR1, IR2, . . . IRn) of said elementary converters, inorder to balance said supply currents (IR1, IR2, . . . IRn), and whichcan implement the method according to claim 1, wherein it comprisesfrequency generators (21) which generate switching frequencies (F1, F2,. . . Fn) for said elementary converters according to the said supplycurrents (IR1, IR2, . . . IRn).
 6. Device for controlling a multiphaseresonant DC/DC converter according to claim 5, wherein it additionallycomprises: a comparator between an output voltage (Vo) of saidmultiphase converter and a nominal voltage (Vref); a regulation loopwhich sets said supply currents (IR1, IR2, . . . IRn) to a commonreference intensity (Iref).
 7. Device for controlling a multiphaseresonant DC/DC converter according to claim 5, wherein said intensitymeasurement means (13) comprise: shunts (12) inserted in series insupply circuits of said elementary converters; voltage measurement means(13) which determine differences of potential (VR1, VR2, . . . VRn) atthe terminals of said shunts (12).
 8. Multiphase resonant DC/DCconverter comprising a plurality of identical elementary resonant DC/DCconverters connected in parallel, wherein it comprises a control device(23) according to claim
 5. 9. Multiphase resonant DC/DC converteraccording to claim 8, the wherein said elementary converters are of theLLC type, each comprising two inductive resistors (1, 2) and a capacitor(3).
 10. Multiphase resonant DC/DC converter according to claim 8,wherein switching means (4, 5) of each of the said elementary convertersare constituted by switching elements (4, 5) connected in the form of ahalf-bridge.
 11. Multiphase resonant AC/DC converter, wherein it isconstituted by an AC/DC converter (24) at the input coupled to amultiphase resonant DC/DC converter according to claim 8 at the output.12. Method for controlling a multiphase resonant DC/DC converteraccording to claim 2, wherein said supply currents (IR1, IR2, . . . IRn)are determined by measuring differences of potential (VR1, VR2, . . .VRn) at the terminals of shunts (12) inserted in series in supplycircuits of said elementary converters.
 13. Method for controlling amultiphase resonant DC/DC converter according to claim 2, wherein saidswitching frequencies (F1, F2, . . . Fn) are derived from avoltage—frequency conversion (21).
 14. Method for controlling amultiphase resonant DC/DC converter according to claim 3, wherein saidswitching frequencies (F1, F2, . . . Fn) are derived from avoltage—frequency conversion (21).
 15. Device for controlling amultiphase resonant DC/DC converter comprising a plurality of identicalelementary resonant DC/DC converters connected in parallel, of the typecomprising intensity measurement means (13) for each of the supplycurrents (IR1, IR2, . . . IRn) of said elementary converters, in orderto balance said supply currents (IR1, IR2, . . . IRn), and which canimplement the method according to claim 2, wherein it comprisesfrequency generators (21) which generate switching frequencies (F1, F2,. . . Fn) for said elementary converters according to said supplycurrents (IR1, IR2, . . . IRn).
 16. Device for controlling a multiphaseresonant DC/DC converter comprising a plurality of identical elementaryresonant DC/DC converters connected in parallel, of the type comprisingintensity measurement means (13) for each of the supply currents (IR1,IR2, . . . IRn) of said elementary converters, in order to balance saidsupply currents (IR1, IR2, . . . IRn), and which can implement themethod according to claim 3, wherein it comprises frequency generators(21) which generate switching frequencies (F1, F2, . . . Fn) for saidelementary converters according to said supply currents (IR1, IR2, . . .IRn).
 17. Device for controlling a multiphase resonant DC/DC convertercomprising a plurality of identical elementary resonant DC/DC convertersconnected in parallel, of the type comprising intensity measurementmeans (13) for each of the supply currents (IR1, IR2, . . . IRn) of saidelementary converters, in order to balance said supply currents (IR1,IR2, . . . IRn), and which can implement the method according to claim4, wherein it comprises frequency generators (21) which generateswitching frequencies (F1, F2, . . . Fn) for said elementary convertersaccording to said supply currents (IR1, IR2, . . . IRn).
 18. Device forcontrolling a multiphase resonant DC/DC converter according to claim 6,wherein said intensity measurement means (13) comprise: shunts (12)inserted in series in supply circuits of said elementary converters;voltage measurement means (13) which determine differences of potential(VR1, VR2, . . . VRn) at the terminals of said shunts (12). 19.Multiphase resonant DC/DC converter comprising a plurality of identicalelementary resonant DC/DC converters connected in parallel, wherein itcomprises a control device (23) according to claim
 6. 20. Multiphaseresonant DC/DC converter comprising a plurality of identical elementaryresonant DC/DC converters connected in parallel, wherein it comprises acontrol device (23) according to claim 7.